it's an 85, the tach is totally different from earlier models. sometimes it's the ovp relay... not sure how to troubleshoot one.
heat problem sounds like a torn monovalve diaphram, click buy parts above and order a repair kit.
glow light could simply be a few bad plugs, go to
www.dieselgiant.com and dig in there for the mercedes maintenance page and go to the glow plug repair page... pretty simple testing instructions there, also, you will need a digital volt ohm meter.